1. Field of the Invention
The present invention relates to apparatus for generating optical pulses primarily of a desired optical wavelength, which can be continuously tunable over a wavelength band.
2. Background of the Invention
Various mechanisms have been proposed to generate wavelength-tunable optical short pulses. The light source that can be used for such purpose includes fiber laser and semiconductor laser diode. The fiber laser supports a continuous wavelength tuning in a wide wavelength band and at a high repetition frequency and therefore is one of the favorable choices. However, its stability is liable to small environmental changes, especially when a long laser cavity is involved. Some other conventional designs use the distributed feedback (DFB) laser diode, which exhibits stable laser operation with continuous wavelength tuning capability. However the DFB laser diode has very narrow tuning range, in other words, the tunable wavelength band is limited, typically less than 2 nm. In addition, both the fiber laser and the DFB laser diode are expensive, which may not be desirable.
Fabry-Perot (FP) laser diode is another possible choice for its relatively low cost, larger wavelength tuning range as compared to the DFB laser diode, and better operation stability as compared to the fiber laser. However, owing to the cavity structure of the FP laser diode, conventionally, the tuning of the wavelength of the output short pulses is usually implemented on a mode-to-mode basis, i.e., the smallest wavelength tuning is limited by the mode spacing of the FP laser diode. This essentially limits the application of the FP laser diode in optical fiber communication in that the mode of the FP laser diode may not coincide with one of the ITU grid wavelengths. Although a smaller wavelength shift or tuning than the mode spacing of the FP laser diode can be achieved by use of thermal control, such an operation can be affected by mode hopping when a large wavelength tuning range is required.